Single phase synchronous run,shaded pole electric motor



NOV. 17, 1970 A DEMlNG ET'AL 3,541,367

SINGLE PHASE SYNCHRONOUS RUN, SHADED POLE ELECTRIC MOTOR Filed Feb. 21, 1968 3 Sheets-Sheet 1 Fig. 3 53 0 5 mm a M W T E NE% W W T Wham W4 Nov. 17, 1970 DEMING ETAL 3,541,367

SINGLE PHASE SYNCHRONOUS RUN, SHADED POLE ELECTRIC MOTOR 3 Sheets-Sheet 2 Filed Feb. 21. 1968 United States Patent Ofice 3,541,367 SINGLE PHASE SYNCHRONOUS RUN, SHADED POLE ELECTRIC MOTOR Andrew F. Deming and Leslie M. Marderwald, Alliance, Ohio, assignors to Alliance Manufacturing Company, Inc., a corporation of Ohio Filed Feb. 21, 1968, Ser. No. 707,124 Int. Cl. H02k 17/26, 21/00 U.S. Cl. 310-114 19 Claims ABSTRACT OF THE DHSCLOSURE a high ratio of pull-in torque relative to the pull-out torque.

The foregoing abstract is merely a rsum of one general application, is not a complete discussion of all principles of operation or applications, and is not to be construed as a limitation on the scope of the claimed subject matter.

BACKGROUND OF THE INVENTION In phonograph motors in recent years there has been increased interest in synchronous motors to drive the turntable in order to achieve an exact speed of reproduction of the sound recorded on the phonograph record. The typical synchronous reluctance motor as used in clock motors, for example, has an extremely low starting and running torque, generally not satisfactory for use in driving phonograph turntables. If the starting torque and driving torque is increased this increases the noise and rumble level in the phonograph mechanism. The rumble includes both 60 Hz. and 120 Hz., for an alternating volt age source of 60 Hz. With the continued interest in greater fidelity it is imperative that the rumble and noise level be kept low and this has created serious problems in attempting to establish sufiicient starting and driving torque with a sufficiently low noise level. Costly solutions to this problem have been suggested but they have not been practical for mass-produced reproducing phonograph players, and have only found use in the most expensive professional turntables such as are used for broadcasting and record cutting.

Additionally, where the phonograph motor is to be used in a record changer, as distinguished from a record player, the motor must supply suficient torque to drive the record changing mechanism which usually incorporates cams or other devices taking considerable load torque. In such case the small synchronous reluctance clock-type motor has been found totally unsatisfactory.

One approach to the solution for a simple and relatively inexpensive phonograph motor was to use a composite rotor having a permanent magnet section plus an induction motor section with a squirrel cage and permeable laminations. The section with the squirrel cage provided the induction motor action for starting and initial acceleraton to an induction motor running speed and the permanent magnet section provided a pull-in to a synchronous running speed. However, such motors had difiiculty in being fabricated at sufliciently low cost to attract the phonograph industry to adopt this type and additionally 3,541,367 Patented Nov. 17, 1970 had other disadvantages of possible demagnetization of the magnets or at least partial demagnetization to lose strength and hence lose synchronous running torque during the life of the phonograph motor. Additionally, ceramic magnets were attempted to be used but these were sufficiently hard that they could not be drilled to provide the requisite dynamic balancing which typically was dynamic balancing to within 0.00025 maximum ounce inch of unbalanced torque. This necessitated careful balancing of the rotor itself plus balancing of the permanent magnet and of the composite rotor. In using a ceramic magnet which could not be drilled, some other softer material on the end of the magnet had to be supplied so that it could be drilled for dynamic balancing and this added to the cost and complexity of the entire motor. Further in the old style of a ceramic magnet, when a softer material was sup plied on the ceramic magnet and it in turn was cemented or adhesively bonded to the induction motor section of the rotor, the adhesive bonding could be broken loose during drilling for the dynamic balancing. Assuming an alternating voltage source of 60' Hz., then 3600 r.p.m. is the synchronous speed for a two-pole motor. Such a two-pole motor is the simplest to construct and hence the most economical. It was found that with the old style of composite rotor using an ordinary squirrel cage section, the induction motor action no load running speed was in the order of 3450 r.p.m. maximum to as low as 3330 r.p.m. With this low an induction motor running speed, the pull-in torque was severely limited because the two-pole permanent magnet had a high slip speed of to 270 r.p.m. relative to the 3600 r.p.m. synchronous speed. Because of this high slip, the pull-in torque was severely limited.

The prior art attempts to increase the pull-in torque to the synchronous run condition included making the motor bigger. This increased the mass and inertia of the rotor, which compounded the problem, because the low induction motor running speed, caused by the high slip, meant that the permanent magnet had more work to do to rapidly accelerate the rotor to the synchronous speed condition.

Where the motor is driving a phonograph turntable, the load on the motor is an appreciable one because the motor is sized to the job to be done in order to produce an economical motor. At the instant of attempted pull-in, then the turntable and record thereon must be accelerated so that the synchronous pull-in torque must supply sufficient torque to drive the record turntable and also to accelerate it at that instant.

In analyzing the problem it was found that the torque was limited by the friction in the bearings which was being increased because the induction motor permeable rotor section was placed on top, hence the solenoid action of the permeable rotor relative to the field structure pulled the rotor down against the thrust bearing. Also where steel mounting plates for supporting the phonograph motor were used, flux was shunted through this steel mounting plate to rob useful flux from the motor and limit the torque. Still further with single-phase motors,

the flux is essentially an alternating flux, not a circular rotating flux to produce a circular field, and is an elliptical field produced by the shaded pole sections to give a starting torque. This non-circular field would further limit the induction motor running speed below 3500 r.p.m.

One attempt by the phonograph industry was to use two synchronous reluctance motors of the clock type to pro vide suflicient torque for driving the phonograph turntable at synchronous speed but this still was not suflicient torque for starting or for changing the records, and an attempt was made to include a third motor of the induction motor type to provide sutficient torque for this starting and record changing. This obviously became a very expensive and hence uneconomical solution to the problem.

If in using a composite rotor of induction motor and permanent magnet sections, an attempt is made to increase the excitation on the single-phase winding, then this has the bad effect of increasing the rumble at 120 Hz. for a 60 Hz. excitation source. The reason is that the magnetic field has a frequency twice that of the alternating voltage source and the increased field increases the non-circularity of the field. This non-circularity is considered the reason for the increased rumble at 120 Hz. Additionally, the prior art used the typical induction motor sections which had skewed rotor bars and open slots in the permeable laminations to receive such rotor bars. These decreased the induction motor running speed and hence decreased the available pull-in torque. Still further the typical induction motor rotor section which was used had a relatively small amount of copper or other conductive material relative to the amount of permeable laminations. This was satisfactory when the rotor was used only as an induction motor but has been found to be the wrong approach when used in a composite rotor to give induction starting and synchronous run characteristics.

The invention may be incorporated in an electric motor comprising, in combination, a permeable field structure, winding means on said field structure to establish an alternating flux in said field structure, a composite rotor journalled for rotation in said field structure, a first and a second section in said rotor, said first section being an induction motor action section including permeable laminations and including a squirrel cage with conductive end rings and rotor bars, said conductive end rings on said squirrel cage having a combined axial thickness at least 80% of the axial thickness of said permeable laminations to have a very low resistance of the squirrel cage relative to the resistance of said permeable laminations, said second section of said rotor including a permanent magnet located axially adjacent said first section of said rotor, said low resistance squirrel cage establishing an induction motor action no-load speed of approximately of the synchronous speed to increase the pull-in torque and establish a synchronous run condition of said rotor.

Other objects and a fuller understanding of this invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plan view of a phonograph motor incorporating the invention;

FIG. 2 is a front elevation of the motor of FIG. 1;

FIG. 3 is a partial sectional view of the motor of FIG. 1 taken on line 33 of FIG. 4;

FIG. 4 is a sectional view of the motor taken on the line 4--4 of FIG. 2; and

FIGS. 5, 6 and 7 are graphs of operating characteristics of the motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT The hereinafter described preferred embodiment is not to be taken as limiting on the invention, the invention only being limited by the hereinafter appended claims.

The FIGS. 1, 2, 3 and 4 show aphonograph motor 11 which incorporates the invention. This phonograph motor 11 has a permeable field structure 12, winding means 13, a composite rotor 14 and bearing means 15 to journal the rotor 14. The field structure 12 establishes a two-pole magnetic field, as best shown in FIG. 4, which is excited by the single-phase Winding 13. The field structure 12 has shading coils 16 to establish a shaded section of each of the two poles to establish a starting torque for the rotor 14. The motor 11 is dependably supported from a mounting plate 17 which is made of some non-magnetic material, for example, aluminum.

The phonograph motor 11 is shown as one providing plural speeds to a turntable 18 indicated partially in FIG. 1. The motor rotor has a shaft 19 journalled in the bearings 15 and the upper end of this shaft 19 has a plurality of steps 20. An idler wheel 21 engages any selected one of these steps 20 and also the inner dependent rim of the turntable 18. The idler wheel 21 is moved vertically by a cam mechanism 22 for selected engagement with any one of the plural steps 20. The FIG. 3 shows the two bearings 15 which journal the shaft 19 vertically and the lower bearing 15 is a thrust bearing to carry the weight of the rotor and shaft.

The FIG. 3 also shows the composite rotor 14 which includes first and second sections 25 and 26. The first section 25 is an induction motor action section and the second section 26 is a permanent magnet section. The first section 25 includes a group of permeable laminations 28 and a squirrel cage 29. The squirrel cage 29 includes endconductive discs 30 and 31 plus conductive rotor bars 32, the ends of which are staked to the conductive discs 30 and 31 and are visible in FIG. 3. The rotor bars 32 pass through closed slots or apertures in the permeable laminations 28. Also in this preferred embodiment the rotor bars 32 are parallel to the shaft axis rather than being skewed as is conventional induction motor practice. In this preferred embodiment the total axial thickness of the permeable laminations 28 is 0.150 inch and the axial thickness of each of the end rings or discs 30 and 31 is 0.100 inch. This makes a total axial thickness of the end rings approximately 133% that of the axial thickness of the permeable laminations 28, which together with the conductive rotor bars 32 provides a very low resistance rotor relative to the resistance of the permeable laminations 28, and relative to conventional induction motor practice. With this ratio of conductive material to permeable laminations, and with this conductive material being copper, it is found that the weight and volume of the copper squirrel cage 29 is greater than the weight and volume of the permeable laminations 28. This extremely low resistance squirrel cage 29 provides a very high noload induction motor action running speed of the rotor 14, as tested with the magnet 26 dernagnetized.

The second section 26 of the rotor 14 is the permanent magnet section and includes a cylindrical permanent magnet which is magnetized in this preferred embodiment with two poles. The resultant cooperation between the magnet 26 and the two-pole field structure '12 is such as to provide a synchronous running speed of 3600 rpm. for a 60 Hz. alternating voltage supplied to the winding 13. Both the first and second sections 25 and 26 of the rotor 14 are cylindrical and have substantially the same air gap with the field structure 12. The permanent magnet 26 is preferably constructed of a rubber-like material to be soft enough to be drilled into the exposed axial end face of the magnet 26 to provide dynamic balancing. The magnet 26 may be provided by permeable small particles held in or bonded into a rubber-like material so that the entire mass may be permanently magnetized and will have a remanence to establish a permanent magnet. In this preferred embodiment the magnet 26 has an axial dimension of 0.250 inch. The composite rotor 14 has an axial thickness greater than the composite rotor 14 with the field structure 12 lying laterally adjacent approximately of the axial thickness of the permanent magnet 26. This means that approximately ,4, of an inch of the magnet extends axially beyond the field structure 12, and this has been found to provide the maximum no-load induction motor action running speed. The field structure 12 lies laterally adjacent all of the permeable laminations 28 of the rotor first section 25 and laterally adjacent the upper end ring 31 plus laterally adjacent about /8 of the permanent magnet 26. This configuration has been found to provide the maximum no-load induction motor action running speed.

In this preferred embodiment the permanent magnet 26 is located physically at only one end of the laminated rotor section 25 and as shown is vertically above this laminated section 25. By this construction the center line of the permeable laminations 28 lies vertically below a lateral center line 33 of the field structure 12, which center line 33 is perpendicular to the axis of the shaft 19. With this construction, the solenoid action of the rotor laminations 28, as the winding 13 is energized, attempts to pull the laminations 28 toward this lateral center line 33 of the field structure 12. The net result of this solenoid action is a tendency to lift the rotor 14 which reduces the effective weight on the lower thrust bearing 15 adn thus reduces the frictional bearing losses to improve the induction motor action no-load speed.

The motor 11 is usable in high quality phonograph motor drives even though the motor is economical to produce. To achieve this high quality, the rotor 14 may be dynamically balanced to within 0.00025 ounce inch of unbalanced torque. This may be accomplished by dynamically balancing the laminated first section of the rotor 25, with balancing holes drilled on both axial ends thereof. Next the rubber-like magnet material 26 may be cemented to one end of the rotor section and again the entire composite rotor 14 dynamically balanced by drilling on the exposed axial end face of the rubber-like magnetic material 26. This rubber-like material may next be permanently magnetized, preferably so that the hole of the dynamic balancing is on a radiall ine from the shaft 19 which radial line is perpendicular to the north-south direction of the permanent magnetism in the magnet 26.

OPERATION FIG. 6 shows a curve of speed in r.p.m. vs. torque in ounce inches. This curve 37 is not a plot of speed vs. torque to a constant load, instead it is obtained by variably loading the rotor shaft on a prony brake or dynamometer device to determine the torque at the various speed settings. This curve 37 shows that the starting torque is in excess of .3 ounce inch and increases slightly to the 1200 r.p.m. speed point whereat the rotor does exhibit a tendency to attempt to lock in at this sub-harmonic synchronous speed. This jump in the curve is shown at numeral 38. The torque increases again to a maximum of about .575 ounce inch at a speed of about 2600 r.p.m. The torque then falls off in the characteristic induction motor running speed curve until another discontinuity 39 is reached at about 3300 r.p.m. This is caused by the permanent magnet 26 which attempts to cause pull-in, however, as soon as the pull-in starts, the induction motor action torque decreases and the dynarnometer load at that point is sufficiently high to prevent complete pull-in to synchronism. Once this discontinuity point 39 is passed by reducing the load torque requirements, the speed-torque curve 37 becomes erratic to the pull-in point 40 at the synchronous speed of 3600 r.p.m. This pull-in torque point 40 is at about 0.125 ounce inch.

The motor 11 may be designed to operate a phonograph turntable requiring a driving torque of approximately 0.075 ounce inch and hence the motor 11 would give at least a safety factor of excess torque to drive the turntable 18 at synchronous speed. The FIG. 6 also shows that the load will stay in synchronism until a pullout torque point 41 is reached at which time the speed will drop until the pull-out torque curve 42 merges with the curve 37 at about 3350 r.p.m. The motor 11 will then sustain a much greater load acting as an induction motor and this greater output torque may be profitably utilized, for example, if the phonograph motor 11 is used in a record changer wherein the record changing cycle requires considerably higher torques than the running torque of just the turntable 18 alone. It will be noted that the pull-in torque point is approximately /3 of the pullout torque point 41, and this is an exceptionally high ratio of pull-in to pull-out torque, especially where the excitation of the winding means 13 is such that the field structure 12 is operated below the knee of the saturation curve. FIG. 6 also shows another curve 44 of the typical prior art curve wherein the pull-out torque point 41 remains the same yet the pull-in torque point 45 is considerably less than 50% of the pull-out torque point 41, and is only about /3 of the pull-in torque 40 achieved by applicants present invention.

FIG. 5 illustrates a family of curves 50-59 of pull-in and pull-out torque vs. applied excitation voltage. This family of curves 50-59 show torque plotted against the excitation voltage on the winding 13. Curves 50-54 are curves of pull-out torque plotted against excitation voltage and curves 55-59 are pull-in torque plotted against excitation voltage. These curves 50-59 show the effect of shading coil size, namely wire gauge of the shading coils 16. FIG. 4 shows that two such shading coils 16 are used on each pole. Curves 50 and 55 are using wire gauge sizes 9 /2 and 9 /2. Curves 51 and 56 are using wire gauge sizes 9 and 9. Curves 52 and 57 are using wire gauge sizes 8 /2 and 8 /2. Curves 53 and 58 are using wire gauge sizes 10 and 10, and curves 54 and 59 are using wire gauge sizes 10 and 13. In typical induction motor practice the two shading coils are made of different size wire, with the shading coil circumscribing a greater extent of the pole piece being of smaller gauge, that is, a higher gauge number of wire. Using wire gauge of 10 and 13 was common practice in prior art induction motors and the curves 54 and 59 show the pull-out and pull-in torque respectively of such a composite rotor used with these wire gauges. In general the pull-out and pull-in torque was improved with increased gauge of wire up to 9 /2 gauge for each of the two shading coils. This is shown in curves 50 and 55. However, increasing the shading coil size to number 9 gauge on each gave a reduced torque for both the pull-in and pull-out and hence in this preferred embodiment the number 9 /2 gauge wire for both shading coils on a pole piece is preferred. Also in the case of the 9 /2 wire gauge shading coils of curves 50 and 55 it is found that the induction motor action no-load running speed, with the magnet 26 removed, is about the highest of all curves, namely 3475 r.p.m.

By the use of the non-magnetic motor mounting plate 17, there is no flux shunted through this mounting plate to rob the rotor 14 of its operating flux and this increases the induction motor action no-load running speed. By placing the magnet 26 on top, the solenoid action on the permeable laminations 28 tends to lift the entire rotor 14 toward the center line 33 of the field structure 12 to reduce the frictional load on the lower thrust bearing 15 and thus also increase the induction motor action no-load running speed. The high ratio of axial thickness of the copper end rings 30 and 31 relative to the axial thickness of the permeable laminations 28 establishes a very low resistance rotor which also achieves a very high induction motor action no-load running speed.

FIG. 7 is a plot of a curve 62 of speed versus a volume ratio of conductor to iron or permeable laminations in the induction motor rotor section 25. This curve 62 of FIG. 7 shows the maximum no-load induction motor running speed of the rotor 25 alone without the magnet 26, or with the magnet 26 present in the composite rotor 14, but unmagnetized. The reason why the magnet 26 has to be unmagnetized or absent is because if it were present then this maximum induction motor running speed would not be capable of ascertainment, because the motor would pull-in to synchronism as shown by the pull-in point 40 of curve 37 in FIG. 6.

The curve 62 is obtained by taking a plot of many different motors of different kinds of rotors with varying ratios of conductor to permeable laminations in the rotor section 25. As more conductive end rings, for example of copper, are provided on the rotor stack, the fewer permeable laminations of steel for example, are used for a given stack height. The typical induction motor rotor is one which would have a ratio of about .4 to .7 of conductor to iron, by volume, and the curve 62 shows that such motor would have no load running speed in the order of 3300 to possibly as high as 3400 r.p.m. This low running speed is because of the relatively high resistance squirrel cage caused by a minimum amount of copper or other conductor. However, the curve 62 shows that when one reaches a ratio of about 1.0 of conductor to iron, the noload speed is up in that range which applicant has discovered to be very desirable, namely about 3480 r.p.m. or above. The reason why this is so desirable is that with the higher no-load operating speed the rotor gets up to a speed whereat the permanent magnet 26 will easily pull the rotor into synchronism at 3600 r.p.m. Much depends upon the efiiciency of the joint between the rotor bars and the end rings, and it has been found that there is a considerable variation in the ordinary production induction motor rotor which results in considerable variation in resistance and hence considerable variation in the no-load running speed. For the ordinary rotor this is at about 3300 to 3400 r.p.m. range. The preferred embodiment is one wherein the volume ratio of conductor to permeable laminations is about 1.83 as shown by the last point on the curve 62 and this ratio by volume is achieved by an axial length ratio of the copper to the iron of 1.33 to 1. This 1.33 ratio is achieved by two copper end discs on each end, each 0.050 inch thick for a total axial length of 0.200 for the copper. The iron laminations are six in number, each 0.025 inch thick for a total axial thickness of 0.150 inch.

The above examples are with a field structure stack of laminations of A)" height. If the total effective length of the composite rotor 14 is kept at about in order to cooperate with this field structure 12, then as one increases the ratio of conductor to iron, this means that more copper end rings are supplied and fewer permeable laminations are supplied. Carrying this to an extreme, one finds that the curve 62 will no longer be generally asymptotic to the 3600 r.p.m. synchronous speed as shown by the extrapolation 63 of curve 62, instead it will fall rapidly as shown by the dotted portion 64. The reason for the rapid drop off portion 64 is because with fewer laminations in the rotor these laminations rapidly become saturated trying to carry sufiicient flux. As an example, with a ratio of 3.33 to l of conductor to iron and using only two laminations for a total of 0.050 inch axial thickness of iron, the speed was down to 2900 r.p.m. Also the rotor weighs about the same so that the windage and friction losses would be about the same and with these saturated rotor laminations, the no-load induction motor action speed is down considerably from the desired speed of about 3500 r.p.m.

This establishes a rotor 14 which has a low mass relative to the pull-in torque. This achieves a high no-load induction motor action running speed of about of the synchronous speed and also achieves a high ratio of pull-in to pull-out torque, namely approximately Utilizing the same motor and placing the magnet on the top rather than on the bottom, the motor actually has a higher pull-in torque. This shows that the frictional drag in the lower thrust bearing actually decreases the pull-in torque. With the magnet on the top, the rotor 14 tends to lift off the bearing. With the magnet on the bottom, the rotor is actually pulled down to the bearing and hence the axial thrust is twice what one would expect and hence this afiects the no-load running speed. The closed slots rather than open slots in the permeable laminations and also the axially parallel rotor bars both improve the no-load running speed. Current practice for single phase shaded pole induction motors is to use an excitation which is considerably above the knee of the saturation curve. This heavy excitation increases the noise and especially the rumble. In this preferred embodiment the excitation on the winding 13 is such that the field structure is excited below the knee of the saturation curve. The low excitation gives a low rumble at Hz., with 60 Hz. excitation voltage. Another advantage of the present construction is that the low resistance of the squirrel cage gives the highest no-load running speed and also the lowest amount of solenoid action pulling the rotor into the center of the field structure 12. An explanation of this is that one has observed a conductive ring placed in a heavy and concentrated field and such field will actually throw this conductive ring out of the field, or else will repel and support it in the air above any physical support structure in defiance of gravity. Conversely, if a magnetically permeable ring is placed in this heavy field it will be attracted to the core concentrating the field. Now because the composite rotor 14 has a much smaller amount of permeable laminations and a much greater amount of conductive squirrel cage than the ordinary induction motor rotor, the conductive ring effect is increased where it is attempted to be thrown out of the field and the solenoid action of attraction to the center line is reduced, thus reducing the overall total solenoid action and producing the highest no-load running speed.

The permanent magnet 26 is shown as having an axial dimension of 0.250 inch. This has been found superior to a thinner magnet of about inch thickness and one theory as to why this is so is because the field established by the field structure 12 is not circular. Assume for the moment that this was a circular field, such as is established in many three-phase motors. With the circular field and no load on the rotor, the permanent magnet 26 would establish the rotor directly in phase with the rotating field. As the load is increased, the rotor slips backward in phase to carry this increased load but is still in synchronism. Now with a weaker magnet such as would be imposed with a thinner magnet, the phase slippage is more. Now take the actual case of the present motor which is not a circular field because it is not three phase, instead it is a shaded pole field with the shaded section being weaker than the main pole flux. At these weaker shaded sections the weaker magnet slips even more from its synchronous position and then pulls back in closer to proper phase position at the main pole section. This is a modulation of the angle of phase lag, especially as the load torque nears the pull-out torque point 41. As this load torque requirement increases there is more flutter of the phase position and this shows up as 120 Hz. rumble in the phonograph. Now with a thicker magnet and hence a stronger magnet, there is not as much modulation of the angle of phase lag and hence there is less total rumble in the phonograph motor 11. If the shaft is bent or the rotor is out of balance, then this provides rumble at 60 Hz., with a 60 Hz. energizing voltage. However, the magnetization is at 120 Hz. and it has been found that the thicker magnet reduces the rumble at 120 HZ. hence the reasoning is that this thicker magnet does definitely affect the total flux available for operation of the motor 11.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An electric motor comprising, in combination,

a permeable field structure,

winding means on said field structure to establish an alternating flux in said field structure,

a composite rotor journalled for rotation in said field structure,

a first and a second section in said rotor,

said first section being an induction motor action section including permeable laminations and including a squirrel cage with conductive end rings and rotor bars,

said conductive end rings on said squirrel cage having a combined axial thickness at least 80% of the axial thickness of said permeable laminations to have a very low resistance of the squirrel cage relative to the resistance of said permeable laminations,

said second section of said rotor including a permanent magnet located axially adjacent said first section of said rotor,

said low resistance squirrel cage establishing an induction motor action no-load speed of approximately of the synchronous speed to increase the pull-in torque and establish a synchronous run condition of said rotor.

2. A motor as set forth in claim 1 wherein said rotor has a resistance sufficiently low to establish a pull-in torque at least two-thirds of the pull-out torque.

3. A motor as set forth in claim '1 wherein said rotor is an induction start synchronous run motor having a single phase winding means and shading coils on said field structure to establish a starting torque.

4. A motor as set forth in claim 1 wherein said permeable field structure has only two poles.

5. A motor as set forth in claim 1 wherein said permanent magnet has only two poles.

6. A motor as set forth in claim 1 wherein said winding means establishes excitation of said field structure below the knee of the saturation curve thereof to establish a relatively low rumble at a frequency twice the frequency of the alternating voltage applied to said winding means.

7. A motor as set forth in claim 1 wherein said permeable field structure has an axial thickness less than the axial thickness of the composite rotor.

8. A motor as set forth in claim 1 wherein said permeable field structure lies laterally adjacent approximately only seven-eighths of the axial thickness of said permanent magnet.

9. A motor as set forth in claim 1 wherein said permeable field structure lies laterally adjacent the permeable laminations of the rotor field section and one of the end rings but not the other of the end rings.

10. An electric motor comprising, in combination,

a permeable field structure,

winding means on said field structure to establish an alternating flux in said field structure,

a composite rotor journalled for rotation in said field structure,

a first and second section in said rotor,

said first section being an induction motor action section including permeable laminations and including a squirrel cage with conductive end rings and rotor bars,

said conductive end rings on said squirrel cage having a combined axial thickness at least 80% of the axial thickness of said permeable laminations to have a very low resistance of the squirrel cage relative to the resistance of said permeable laminations,

said second section of said rotor including a permanent magnet located axially adjacent said first section of said rotor,

said low resistance squirrel cage establishing an induction motor action no-load speed of approximately of the synchronous speed to increase the pull-in torque and establish a synchronous run condition of said rotor,

a thrust bearing in said motor journalling said composite rotor with the axis thereof vertical,

said motor being adapted to drive a phonograph turntable,

a mounting plate disposed above the motor composed of non-magnetic material to avoid shunting flux from the field structure,

and means dependently supporting said motor from said mounting plate.

11. A motor as set forth in claim 10 wherein said permanent magnet rotor section is disposed vertically above said rotor first section so that the permeable laminations of the rotor are below a lateral center line of the field structure such that solenoid action tends to lift the laminated rotor section toward said lateral center line to reduce the effective weight of the rotor on said thrust bearing.

12. A motor as set forth in claim 1 wherein said permanent magnet is of a ru'bber-like material cemented to one axial end of said rotor first section and being sufiiciently soft to be drilled for dynamic balancing.

13. A motor as set forth in claim 1 wherein said conductive end rings in said squirrel cage having a combined axial thickness greater than the axial thickness of said permeable laminations to es ablish a very low resistance of the squirrel cage.

14. A motor as set forth in claim 1 wherein said squirrel cage has a volume at least of the volume of said permeable laminations in said rotor.

- 15. A motor as set forth in claim 1 wherein said rotor first section has closed apertures therein receiving said rotor bars to increase the induction motor action running speed of the rotor.

16. A motor as set forth in claim 1 wherein said rotor permeable laminations are low in inertia relative to the squirrel cage to establish a composite rotor with low total inertia and consequently a high induction motor action running speed for increased pull-in torque.

17. A motor as set forth in claim 1 wherein said rotor bars of the squirrel cage are axially parallel.

18. A motor as set forth in claim 1 including shading coils on said field structure to establish a shaded section therein, said shading coils being of large cross-sectional area to increase the pull-in torque of said motor from the induction running speed to synchronous speed.

19. A motor as set forth in claim 1 wherein said motor is an induction start, synchronous run motor,

said field structure having only two poles,

said winding means being single phase and energizable from a single phase alternating voltage source,

a mounting plate disposed above the motor composed of non-magnetic aluminum material to avoid shunting of flux from the field structure and magnet,

means dependently supporting said motor from said mounting plate,

a shaft in said rotor,

a thrust bearing in said motor at the bottom of said rotor to support the rotor and shaft vertically thereabove,

said first rotor section having a cylindrical outer surface and having a given air gap with said field structure,

closed apertures in said laminated first rotor section to receive said rotor bars,

said two poles of said field structure having a lateral center line perpendicular to the rotor axis,

said rotor permanent magnet having a cylindrical outer surface and having substantially the same air gap with said field structure as said rotor first section,

said rotor second section being located physically above the first section so that the permeable laminations of the rotor are below said perpendicular I center line of the field structure poles such that solenoid action tends to lift the laminated rotor section toward the center line of the field structure to reduce the effective weight of the rotor on said thrust bearing,

said permanent magnet being of a rubber-like material cemented to the top of said rotor first section and i being sufiiciently soft to be drilled for dynamic balancing,

shading coils on said 'field structure to establish a shaded section of the two poles to establish a starting torque,

said winding means establishing relatively low excitation of said field structure to operate the field structure below the knee of the saturation curve.

2,673,939 3/1954 Tetro 310162 2/1963 Volkrodt 310-456 12 v 3,119,941 1/1964 Guidot 310-156 3,209,185 9/1965 Draper 310172 3,413,501 11/1968 Dotto 310172 WARREN E. -RAY, Primary Examiner R. SKUDY, Assistant Examiner U.S.' Cl. X.R. 

